Heretofore, as the exterior covering or sheathing for electronic parts such as capacitors, a heat-shrinkable tube made of polyvinyl chloride has been widely used. However, in the field where miniaturization and surface mounting of electronic parts such as capacitors are advanced to a large extent, the heat resistance is inadequate with such polyvinyl chloride tube.
For the heat-shrinkable tube for exterior covering of electronic parts such as capacitors, the following properties are required.
(a) When an electronic part such as a capacitor is shrink-covered with a heat-shrinkable tube, both open-end portions (sleeve portions) of the tube bend inwardly to cover the end surfaces of the electronic part. Each bent sleeve portion must uniformly and tightly fit on the end surface of the electronic part to present a good appearance. Namely, it is not desirable that the inner edge of the inwardly bent sleeve portion of the shrinked tube becomes bulky or stands up like a horn, or the sleeve portion curls without tightly fit on the end surface, thus presenting a poor outer appearance. PA1 (b) After being applied to an electronic part, the shrinked tube is subjected to heat treatment for heat fixing. It is desired that during such heat treatment, the tube will not undergo cracking, or the tube will not undergo reshrinking. PA1 (c) The discoloration of the tube is little even when the tube is held at 125.degree. C. for 5,000 hours.
These states are illustrated by drawings. FIGS. 1 to 4 illustrate the states when a capacitor was shrink-covered with a heat-shrinkable tube, wherein reference numeral 1 indicates a capacitor, and numeral 2 indicates a cross sectional portion of the shrink-covered tube.
FIG. 1 represents a case of a tube having good finish, and FIGS. 2 to 4 represent cases of tubes having poor finish. In the case illustrated in FIG. 1, the inwardly bent sleeve portions of the shrinkable tube are uniformly fit on the end surfaces of the capacitor. Whereas, in the case illustrated in FIG. 2, the forward edges of the inwardly bent sleeve portions stand up like horns. In some cases, not only the forward edges, but the entire sleeve portions are likely to stand up. In the case illustrated in FIG. 3, the forward edges of the bent sleeve portions became bulky. In the case illustrated in FIG. 4, the bent sleeve portions are inwardly curled without being tightly fit on the end surfaces. In the conventional processings, such poor finish was often observed.
In order to improve the heat resistance, it has been proposed to employ a thermoplastic polyester resin as the material for a heat-shrinkable tube (Japanese Unexamined Patent Publications No. 32972/1974 and No. 100118/1980). However, such a tube does not fully satisfy the requirement (a) although it may satisfy the requirements (b) and (c) along the above requirements (a), (b) and (c). Therefore, it is desired to develop a thermoplastic polyester heat-shrinkable tube (hereinafter this may sometimes be referred to simply as a heat-shrinkable tube) which has these three essential properties.